This invention relates to vacuum pumps and, more particularly, to vacuum pumps comprising or incorporating turbo-molecular pumping stages.
A conventional turbo-molecular stage arrangement of a vacuum pump comprises a stack of alternate rotors and stators. Each stage effectively comprises a solid disc with a plurality of blades depending (nominally) radially therefrom; the blades are evenly spaced around the circumference of the disc and angled "about" radial lines out of the plane of the disc in the direction of rotation of the rotor stage.
The rotor and stator blades have positive and negative gradients respectively when viewed from the side in to a radial line of the disc. This arrangement has the effect in highly viscous flow conditions of causing rapid changes in the flow direction, resulting in high power consumption.
In molecular flow conditions, the performance of a conventional turbo-molecular pump is attributable to certain molecules of gas being pumped by the alternate rotor-stator pairs in the following way:
i) gas at the inlet has random motion, PA1 ii) rotating blades on (say) the inlet stage (rotor) provide a higher transmission probability downwards rather than upwards due to blade angle and relative blade velocity, thereby generating compression, PA1 iii) gas in to the next stage has a velocity component in rotor direction equivalent to rotor velocity, PA1 iv) stationary blades on the next stage (stator) again provides a higher transmission probability downwards than upwards due to blade angle and relative gas velocity, thereby again generating compression, PA1 v) gas exiting the stator stage has no relative velocity, i.e. random motion again. PA1 a) to provide compression from blade angle and relative velocity, and PA1 b) to redirect gas molecules to sustain a relative velocity between the gas and the blades for each stage through the pump. PA1 i) gas at the inlet has purely random motion, PA1 ii) at the first rotor stage, the rotating blades generate a higher transmission probability downwards than upwards due to the blade angle and relative blade velocity and hence, as in conventional designs, generates compression. PA1 iii) gas in to the next (frusto-conical) stage has a velocity component in the rotor direction equivalent to rotor velocity so that when the gas enters the stage--having moved tangentially some distance from the previous rotor--it also has a radial component of velocity. PA1 iv) observed in a diametric section, the stator conical members behave like conventional "radial" blades and provide a relative velocity equal to the radial component of the gas velocity. The effective blade angle and spacing is similar to that used in conventional radial blades. The radial component of the velocity in the conical members provides a higher transmission probability downwards than upwards and thereby generates compression of the gas. PA1 v) gas leaving the stator stage has no relative velocity in the direction of rotation and therefore the gas resumes random motion. PA1 Such an arrangement of the stages and a reverse arrangement of the stages as stator and rotor, requires a significantly reduced power consumption for atmospheric pressure operation but, surprisingly without significant loss of overall performance at lower pressure (higher vacuum) operation. Each stage achieves the two basic previously stated functions required of them, i.e. to provide compression and to redirect molecules.
It should be noted that certain other molecules of gas being pumped, in molecular flow conditions, do not interact with each stage of the pump but pass through some stages unaffected.
If the pump comprised only rotor stages, there would exist no relative velocity between the gas and the rotating blades after leaving the surface of the first rotor and therefore no preferential gas direction through the second (and subsequent) rotors.
Thus a pump consisting solely of rotors (or solely of stators) would generate very little or no compression although power consumption would be reduced dramatically.
Both rotors and stator stages are therefore clearly necessary in a turbo-molecular pump arrangement; the function of each stage is two-fold.
Turbo-molecular vacuum pumps are designed to operate at high rotational speeds of the shaft to which the rotor discs are attached and to achieve high levels of vacuum in the chambers to which they are attached. Turbo-molecular pumps are generally unable to deliver gases directly to the atmosphere; the use of a backing pump of different pumping mechanism which pumps down or "roughs" the pressure in the chamber, preferably prior to the operation of the turbo-molecular pump, and in to the inlet of which the output of the turbo-molecular pump is subsequently directed, is therefore generally needed.
The backing pump may alternatively be incorporated in to the turbo-molecular pump body to form a compound vacuum pump. For example, the turbo-molecular pump stages may be followed, in order of gas flow through the pump as a whole, by one or more molecular drag stages, for example those known as "Gaede" stages or "Holweck" stages, and regenerative stages to exhaust to atmospheric pressure.
A compound design incorporates the different pump stages/mechanisms, the rotors of which are all rigidly mounted on a single shaft and each mechanism being suited to pumping in different vacuum pressure regions. As such, the combination of mechanisms provide a steady pressure gradient through the pump as a whole from inlet to outlet.
The major consideration of a compound design is the electrical power required during the initial pump down. Prior to the pressure gradient being established across the pump, all mechanisms are required to rotate at atmospheric pressure. In this condition, conventional turbo-molecular blades--whether in a simple single mechanism pump or in a compound pump--generate large viscous shear and turbulence effects between the rotors and the stators resulting in high and often impractical levels of power consumption; the faster the pump shaft speed, the greater is the power consumption. Reducing the number of turbo-molecular stages although reducing power consumption would simply adversely affect the pump performance.
Where sufficient shaft speeds can be attained during initial operation of the pump, the mechanisms suited to pumping in viscous flow conditions begin to reduce the upstream pressure in the pump and thereby reduce the power required to rotate the turbo-molecular blades. The shaft speed can then increase and the pressure at the pump inlet can reduce further.
There is a need to minimise the atmospheric pressure power consumption of the turbo-molecular portion of a compound pump or as a part of a turbo-molecular roughing pump system without redress to the simple expedient of reducing the number of turbo-molecular stages.
The invention addresses this need through modified turbo-molecular pump design by substantially or completely eliminating turbulence and viscous shear, thereby allowing an adequate number of turbo-molecular stages to be employed for good pumping performance without the requirement of excessive power consumption.